1. Field of Use
This invention relates generally to web processing apparatus wherein a web moving along a path is supported on a guide by a cushion of air without contacting the guide.
In particular, it relates to apparatus and method for ultrasonically controlling the position of the web to maintain it at a predetermined distance from the guide.
2. Description of the Prior Art
U.S. Pat. No. 4,282,998 entitled "Maintenance of Constant Web Clearance Turning Guide" issued Aug. 11, 1981 to Peekna discloses web processing apparatus wherein a change in direction of a running web is accomplished by means of a contactless running guide. In that guide, pressurized air from a source, such as a blower, is delivered into a plenum chamber from which it issues through slots in the guide that direct it between the web and a curved guide surface to form a pressurized air cushion on which the web floats out of contact with said guide surface. A damper that controls flow of pressurized air into the plenum chamber is automatically positioned by a servomotor so as to maintain a constant distance between the guide surface and the web, notwithstanding variations in web tension. Means for control of the servomotor comprise a supply pressure sensor that produces a first output corresponding to the above-atmospheric pressure of air in the plenum chamber, a cushion pressure sensor having an inlet at said guide surface that produces a second output corresponding to the above-atmospheric pressure in the air cushion, and a ratio-forming device to which said outputs are fed to produce a ratio signal corresponding to a ratio relationship between the first and second outputs. In a comparison device, the ratio signal is compared with a manually adjustable set-point signal to produce a control output signal which is applied to the servomotor so as to maintain said ratio relationship substantially constant.